1. Field of the Invention
The present invention relates to a laser light source apparatus preferable as a light source having a light emitting element which outputs high power laser light to be used for optical fiber lasers, etc., and a pulse generation method of stably generating light pulses in the laser light source apparatus. In particular, the laser light source apparatus is preferable as a laser light source apparatus for a light emitting element of MOPA. In the present application, the light emitting element is a photoelectric conversion light emitting element capable of high power output, such as a LD (Laser Diode) or VCSEL (Vertical Cavity Surface Emitting laser: surface emitting laser).
2. Related Background Art
At present, a processing technique using pulsed laser light which is repeatedly outputted at a predetermined period has attracted attention, and demand for high power laser light sources has increased in the fields of processing and medical treatment. One type of laser light source which has especially attracted a great deal of attention among various laser light sources is optical fiber lasers. The optical fiber laser adopts, as an optical amplification medium, an amplification optical fiber having a core doped with a rare earth element such as Yb (Ytterbium), Er (Erbium), or Tm (Thulium). When pumping light is supplied into the amplification optical fiber, seed light propagating inside the amplification optical fiber is amplified. Accordingly, from the amplification optical fiber, amplified light with high power is outputted, or laser light is outputted by laser oscillation by using a resonator structure. Advantages of the optical fiber lasers are ease in handling due to confinement of laser light within the optical fiber and needlessness of large-scale cooling equipment due to high thermal emissivity.
As described above, rare earth element-doped fibers are applied to the optical fiber lasers, and among these rare earth element-doped fibers, a fiber doped with Yb with high conversion efficiency is widely used as an amplification optical fiber for high power output. Yb is also pumped by using pumping light as in the case of other rare earth elements. On the other hand, pumping light which cannot be absorbed inside the amplification optical fiber is made to exit from the other end of the amplification optical fiber.
As a configuration of the optical fiber laser, for example, when a resonator structure using a Fiber Bragg Grating (FBG) or a reflecting mirror, etc., on both ends is adopted, pulse modulation is performed by disposing an optical switch or an Acoustic Optical Modulator (AOM) inside the resonator. The MOPA (Master Oscillator Power Amplifier) type optical fiber laser described in Japanese Patent Publication Laid-Open No. 2007-042981 obtains high-power output light by amplifying a light pulse obtained by pulse modulation performed by direct modulation or external modulation of a seed light source (light emitting element) which outputs light to be amplified. In each configuration, a peak output obtained by pulsing seed light is much higher than the output in continuous wave operation (CW operation), and causes a nonlinear phenomenon such as Stimulated Raman Scattering (SRS) or Stimulated Brillouin Scattering (SBS).
In the MOPA type optical fiber laser, when an external modulation method is used in which pulse modulation is externally applied to an output light of a seed light source, pulse modulation control (one control period consists of a duration corresponding to the width of the light pulse, set to be in a pulse-ON state, and a duration corresponding to an interval between adjacent light pulses, set to be in a pulse-OFF state) can be performed by controlling a specific voltage value to be applied to an external modulator such as an AOM. Similarly, in the case of a direct modulation method in which direct pulse modulation is controlled for a seed light source, a method in which a voltage of a TTL signal (5 to 0 V) and a low volt TTL signal (3.3 to 0 V) are applied to the seed light source is known as a method of pulsing output light of the seed light source according to a pulse modulation pattern of a driving current to be supplied to the seed light source. Whether the duration of high-voltage application is set to be in the pulse-ON state or the pulse-OFF state depends on the electric circuit design, however, each of these requires a high application voltage, and causes an increase in size of the electric circuit of the modulator and an increase in electric power consumption. When the pulse responsiveness of the modulator itself, specifically, if the rising time and the falling time are late, switching between the pulse-ON state and the pulse-OFF state takes a longer time and delays. As a result, there is a risk that the pulse width (full width at half maximum) of the obtained seed light pulse L becomes narrower. It is also possible to control the specific modulation voltage to be applied to the seed light source when the driving current is changed, however, in this case, there is a possibility that control is not successfully performed or delayed, light pulses with a normal pulse width cannot be obtained, and the amplification optical fiber does not send the seed light pulses. In this case, a large amount of ASE light caused in the amplification optical fiber may damage other optical components (isolator and pumping light source, etc.) and the amplification optical fiber.